The Student Room Group

Volume of a cylinder and sphere?


So I did pi*r2*h for the volume of the cylinder which is 4x2*pi*9x

Then I did that = the volume of the spehere:
4x2*pi*9x = 4/3*4x2*pi

Then

(4/3*4x2*pi) / (4x2*pi) = 9x

1/34x2*pi = 9x

(1/34x2*pi)/9 = x

Right or wrong? Please help! Thanks :smile:
Q.png29.0KB
Original post by Qaiys

So I did pi*r2*h for the volume of the cylinder which is 4x2*pi*9x

Then I did that = the volume of the spehere:
4x2*pi*9x = 4/3*4x2*pi

Then

(4/3*4x2*pi) / (4x2*pi) = 9x

1/34x2*pi = 9x

(1/34x2*pi)/9 = x

Right or wrong? Please help! Thanks :smile:


You do not have the correct formula for the volume of a sphere and you seem to have ignored the r

43πr3\dfrac{4}{3}\pi r^3


Then, I have to say - your algebraic manipulation is very wrong
(edited 9 years ago)
Reply 2
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
You do not have the correct formula for the volume of a sphere and you seem to have ignored the r

43πr3\dfrac{4}{3}\pi r^3


Then, I have to say - your algebraic manipulation is very wrong


So would 4x2*pi*9x = 4/3*8x3*pi be right?
Original post by Qaiys
So would 4x2*pi*9x = 4/3*8x3*pi be right?


No

Why do you have no r in your work
Reply 4
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
No

Why do you have no r in your work


The radius is 2x
Original post by Qaiys
The radius is 2x


The radius of the cylinder is 2x

Not the radius of the sphere
Reply 6
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
The radius of the cylinder is 2x

Not the radius of the sphere


So how do I find the radius of the sphere?
Original post by Qaiys
So how do I find the radius of the sphere?


It tells you in the question that it is r
Reply 8
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
It tells you in the question that it is r


ohhh, oh yeah. Okay so

pi*r^2h = pi*4x^2*9x

pi*4x^2*9x = 4/3pi*r"3

Then rearrange for x?
Original post by Qaiys
ohhh, oh yeah. Okay so

pi*r^2h = pi*4x^2*9x

pi*4x^2*9x = 4/3pi*r"3

Then rearrange for x?


well rearrange for r, since the question asks for r in terms of x

but yes
Reply 10
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
well rearrange for r, since the question asks for r in terms of x

but yes


Oh yeah sorry >.< and thanks! I'll try and post in a sec :biggrin:
Original post by Qaiys
Oh yeah sorry >.< and thanks! I'll try and post in a sec :biggrin:


:biggrin:

Perseverance is the first stage on the route to success
Reply 12
Avatar for white_o
white_o
OP
17
4x^2*pi*9x = 4/3*pi*r^3

(4x^2*pi*9x)/(4/3*pi) = r^3

cube root((4x^2*pi*9x)/(4/3*pi)=r

Would the pi's cancel out?

Thanks
Reply 13
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
:biggrin:

Perseverance is the first stage on the route to success


Haha absolutely :P Thanks!
Original post by Qaiys
4x^2*pi*9x = 4/3*pi*r^3

(4x^2*pi*9x)/(4/3*pi) = r^3

cube root((4x^2*pi*9x)/(4/3*pi)=r

Would the pi's cancel out?

Thanks



The pi does cancel
Reply 15
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
The pi does cancel


So would cube root((4x^2*9x)/(4/3)) = r be the final answer?
Original post by Qaiys
So would cube root((4x^2*9x)/(4/3)) = r be the final answer?


You need to work on your algebra skills

Firstly 4x2∗9x=36x34x^2 * 9x = 36x^3

Then dividing by 43\frac{4}{3} is the same as multiplying by 34\frac{3}{4}
Reply 17
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
You need to work on your algebra skills

Firstly 4x2∗9x=36x34x^2 * 9x = 36x^3

Then dividing by 43\frac{4}{3} is the same as multiplying by 34\frac{3}{4}


Gahh, I'm not usually this terrible I promise, I've been revising allll day for my exam tomorrow


So 36x^3 * 4/3 = 108x*3/4 = 27x^3

so r = 3x?
Original post by Qaiys
Gahh, I'm not usually this terrible I promise, I've been revising allll day for my exam tomorrow


So 36x^3 * 4/3 = 108x*3/4 = 27x^3

so r = 3x?


woo hooooooo :biggrin:

Yes
Reply 19
Avatar for white_o
white_o
OP
17
Original post by TenOfThem
woo hooooooo :biggrin:

Yes


Haha sorry for being so rubbish!! Thanks a lot :biggrin:

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you